In recent years, as electronic devices and telecommunication devices have become smaller in size and lighter in weight, the demand for secondary batteries to be used as a power supply for such devices has been increased. Among various secondary batteries, lithium secondary batteries, because of their high capacity, high energy density, and excellent charge/discharge cycle characteristics, have a significantly large market share. Typical lithium secondary batteries are mainly constituted of a negative electrode including a carbon material capable of absorbing and desorbing lithium, and a positive electrode including a composite oxide of a transition metal and lithium, such as LiCoO2. However, in response to the current trend toward more multi-functional electronic devices and telecommunication devices, with respect to such lithium secondary batteries also, there have been demand for achieving a higher energy density and further improving charge/discharge cycle characteristics.
Under these circumstances, alloy-based active materials that absorb lithium by forming an alloy with lithium have been attracting attention as materials to satisfy such demand. Examples of known alloy-based active materials include silicon, tin, and germanium; and an alloy, an oxide, and a nitride containing these. Such alloy-based active materials have a high capacity. For example, the theoretical discharge capacity of silicon is about 4199 mAh/g, which is about eleven times as large as that of graphite (e.g., see Patent Document 1). However, there is a problem to be solved in such alloy-based active materials, that is, the structure thereof significantly changes during charging when lithium ions are absorbed therein. In an active material layer containing an alloy-based active material (hereinafter referred to as an “alloy-based active material layer”), the stress generated in association with expansion of the alloy-based active material could easily cause disadvantages such as cracks of alloy-based active material particles, separation of the alloy-based active material layer from a current collector, deformation of the current collector followed by deformation of the electrode, and others. These disadvantages will then be the cause of the reduction in the electron conductivity between the alloy-based active material layer and the current collector, and the deterioration in the charge/discharge cycle characteristics of the battery.
In view of the problem in alloy-based active materials, for example, an electrode including a specific current collector and a thin film of alloy-based active material layer has been proposed (e.g., see Patent Document 2). In this proposal, the current collector is made of a copper alloy and has a tensile strength of 400 N/mm2 or more, a proportional limit of 160 N/mm2 or more, an elastic modulus of 1.1 N/mm2 or more, and a surface roughness Ra of the surface on which the alloy-based active material layer is formed of 0.01 to 1 μm. Further, for the purpose of adjusting the surface roughness Ra of the surface of the current collector within the foregoing predetermined range, forming projecting matters made of copper on the surface of the current collector by electrolysis plating is disclosed in Patent Document 2. Further, forming a thin film of alloy-based active material layer on the surface of the current collector by vacuum vapor deposition, plating, and the like is disclosed. The technique disclosed in Patent Document 2 is effective to some extent in terms of successfully increasing the bonding strength between the alloy-based active material layer and the current collector and preventing the separation of the alloy-based active material layer from the current collector. However, merely increasing the bonding strength between the alloy-based active material layer and the current collector cannot sufficiently absorb the expansion stress of the alloy-based active material, and thus the deformation such as wrinkling and cracking inevitably occurs in the electrode.
In order to relieve the expansion stress of the alloy-based active material, techniques of providing gaps in the interior of the alloy-based active material layer have been proposed (see, for example, Patent Documents 3 and 4). Patent Document 3 discloses an electrode comprising a current collector with projections and depressions formed on a surface thereof, and a thin film of alloy-based active material layer formed on the surface of the current collector with projection and depressions provided thereon. When this electrode is incorporated in a battery and the battery is charged for the first time, due to the expansion stress of the alloy-based active material, gaps are formed in the thin film of alloy-based active material layer along the projections and depressions on the surface of the current collector. The technique disclosed in Patent Document 3 intends to allow the expansion stress of the alloy-based active material to be absorbed by the gaps. Disadvantageously, however, since the gaps are formed by charging in Patent Document 3, it is difficult to obtain evenly-distributed gaps in the thin film of alloy-based active material layer. As such, in this thin film of alloy-based active material layer, there are a portion where the expansion stress can be relieved because of the presence of gaps and a portion where the expansion stress cannot be relieved because of the absence of gaps. As a result, sufficient stress-relieving effect cannot be obtained, and thus the electrode may be deformed.
Patent Document 4 discloses an electrode comprising a current collector with a predetermined projection/depression pattern formed on a surface thereof, and an alloy-based active material layer composed of a plurality of columns containing an alloy-based active material. The projection/depression pattern is formed by photolithography. The columns are formed so as to extend from the surfaces of the projections or depressions in the projection/depression pattern of the current collector, in directions normal to the surfaces. Forming columns only on the surfaces of the projections or depressions in the predetermined projection/depression pattern brings about both an advantage and a disadvantage in the technique disclosed in Patent Document 4. The advantage is the presence of gaps between columns adjacent to each other. By virtue of these gaps, the expansion stress in the columns toward the sides thereof is relieved. The disadvantage is that the expansion stress in the columns toward the current collector is concentrated at the interfaces between the columns and the projections or depressions, causing the columns to be easily separated from the surfaces of the projections or depressions. Accordingly, with the technique disclosed in Patent Document 4, the deformation of the electrode is prevented to some extent, but it is difficult to completely prevent the separation of the columns, namely, the alloy-based active material layer.
Moreover, an electrode including a current collector and a thin film of alloy-based active material layer, in which the value of (Surface roughness Ra of thin film of alloy-based active material layer)−(Surface roughness Ra of current collector) is 0.1 μm or more has been proposed (see, for example, Patent Document 5). Conventionally, a thin film is formed on a surface of the current collector by vacuum vapor deposition and the like, which provides a thin film having a surface roughness approximately equal to that of the current collector. In Patent Document 5, a thin film formed by the conventional method is subjected to a treatment, such as sand-blasting and surface-grinding, so that the surface roughness of the thin film is adjusted to the foregoing specific value. By doing this, it is intended to relieve the stress due to expansion of the alloy-based active material. The technique disclosed in Patent Document 5 is effective to some extent in that cracks on the alloy-based active material can be prevented, but has no significant difference from those disclosed in Patent Documents 2 and 3 in that a thin film is formed over the entire surface of the current collector. As such, the separation of the thin film from the current collector, the deformation of the electrode, and the like will easily occur.
In addition, with regard to the technique of roughening the surface of a current collector in order to enhance the bonding strength between the current collector and the active material layer, various proposals have been suggested in patent documents other than the above patent documents. For example, there has been proposed a method of allowing fine particles to be ejected from a nozzle and collide with the surface of a rolled copper foil at high speeds, thereby forming minor projections and depressions on the surface (see, for example, Patent Document 6). According to this method, it is difficult to uniformly form projections and depressions on the surface of a current collector in its longitudinal and lateral directions since there is a variation in the velocity of the fine particles ejected from the nozzle.
Furthermore, there has been proposed a metallic foil surface roughening method of forming projections and depressions by irradiating a metallic foil with laser beams so that the metallic foil has a surface roughness of 0.5 to 10 μm as a 10-point average roughness (see, for example, Patent Document 7). According to the technique disclosed in Patent Document 7, depressions are formed by irradiating a metallic foil with laser beams to locally heat the metallic foil and vaporize metal in the heated portion. The metallic foil, however, will be subjected to heat higher than the melting point of the metal constituting the metallic foil by laser irradiation. Moreover, since the laser beams are linearly applied, the irradiated portions and non-irradiated portions will coexist in the metallic foil. Because of this, it is difficult to prevent the occurrence of crinkling, wrinkling, warping, and the like on the metallic foil. In addition, in the case where a metallic foil having a thickness of 20 μm or less, such as a current collector for a lithium secondary battery, is subjected to laser irradiation, disadvantageously, the metallic foil may be perforated because of the variation in the output power of the laser.
Further, in order to improve the bonding strength and the electric conductivity between the active material layer and the current collector, there has been proposed a current collector having specific projections and depressions (see, for example, Patent Document 8). FIGS. 21(a) to (e) are perspective views schematically showing a configuration of the current collector of Patent Document 8. On the current collector of Patent Document 8, projections and depressions are regularly formed in such a manner that while a local portion on one surface of the metallic foil is depressed, a portion corresponding to the local portion on the other surface of the metallic foil projects outwardly from the other surface. The current collector having such projections and depression, during the production thereof, will unavoidably have deformations such as crinkling, wrinkling, and warping.
Still further, there has been proposed an electrode including: a current collector made of a punching metal having a porosity of 20% or less and having projections and depressions formed by embossing; and a layer made of an active material filling the depressions of the current collector, in which the projections of the current collector are exposed or the active material adheres to the projections (see, for example, Patent Document 9). FIGS. 22(a) to (c) are longitudinal cross-sectional views schematically showing a configuration of electrodes 101 to 103 of Patent Document 9. The electrode 101 shown in FIG. 22(a) includes a current collector 110 with projections and depressions formed thereon and a layer 111 of active material filling depressions 110b of the current collector 110. The active material layer 111 adheres to the surfaces of projections 110a of the current collector 110. In the electrodes 102 and 103 shown in FIGS. 22(b) and (c), projections 120a and 130a of current collectors 120 and 130 are both exposed. According to Patent Document 9, the projections and depressions are formed by embossing the punching metal having a porosity of 20% or less, the obtained current collector fails to have a sufficient mechanical strength. This may disadvantageously result in tearing of the electrode, and the like.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-83594
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-7305
Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-283834
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-127561
Patent Document 5: Japanese Laid-Open Patent Publication No. 2002-279972
Patent Document 6: Japanese Laid-Open Patent Publication No. 2002-79466
Patent Document 7: Japanese Laid-Open Patent Publication No. 2003-258182
Patent Document 8: Japanese Laid-Open Patent Publication No. 2002-270186
Patent Document 9: Japanese Laid-Open Patent Publication No. 2005-32642